Combustion method with cyclic supply of oxidant

ABSTRACT

The invention concerns a combustion method for industrial furnace comprising an arrangement of two substantially parallel and symmetrical burner assemblies (G, D). Each burner assembly comprises a fuel injector ( 10 &lt;SUB&gt;G&lt;/SUB&gt;,  10 &lt;SUB&gt;D&lt;/SUB&gt;) and three oxidant injectors ( 1 &lt;SUB&gt;G&lt;/SUB&gt;,  2 &lt;SUB&gt;G&lt;/SUB&gt;,  3 &lt;SUB&gt;G&lt;/SUB&gt;,  1 &lt;SUB&gt;D&lt;/SUB&gt;,  2 &lt;SUB&gt;D&lt;/SUB&gt;,  3 &lt;SUB&gt;D&lt;/SUB&gt;) arranged at increasing distances from the fuel injector. An oxidant supply system cyclically distributes a specific flow of oxidant among some at least of the second and third injectors of the burner assemblies ( 2 &lt;SUB&gt;G&lt;/SUB&gt;,  3 &lt;SUB&gt;G&lt;/SUB&gt;,  2 &lt;SUB&gt;D&lt;/SUB&gt;,  3 &lt;SUB&gt;D&lt;/SUB&gt;). The amount of nitrogen monoxide produced upon combustion is thus reduced, while ensuring a good distribution of the heating power in the furnace.

This application is a 371 of International PCT ApplicationPCT/FR2005/051033, filed Dec. 5, 2005.

BACKGROUND

The present invention relates to a combustion method for an industrialfurnace. It also relates to a furnace suitable for implementing such amethod.

The heating power distribution on a given furnace surface, the reductionof the quantity of nitrogen oxides produced, and the stability of thecombustion flame(s) generated in the furnace, are among the mainchallenges in combustion furnace technology.

In fact, the energy efficiency and the profitability of an industrialcombustion furnace are higher for large capacity furnaces. This is whythe surface to be heated may be large. This is generally the uppersurface of a charge of raw materials or melt contained in a chamber. Itis accordingly difficult to distribute the heating power delivered bythe combustion flame(s) substantially uniformly over the whole surface,to prevent the formation of colder zones that would be harmful to themelt or to the subsequent method for treatment thereof. For thispurpose, a plurality of burners is known to be arranged in a furnace, atpredefined locations above the chamber. In particular, two burners canbe placed in parallel to one another, with respective horizontal flamesdirected in the same direction. Another alternative is to position theburners in opposing pairs, with the respective flames directed at oneanother within each pair.

Furthermore, the quantity of nitrogen oxides (No_(x)) produced in acombustion flame depends on the local oxygen and nitrogenconcentrations, denoted [O₂] and [N₂]. In particular, an evaluation ofthe quantity of thermally produced nitric oxide (denoted [NO]_(th)) isgiven by the following equation:

$\begin{matrix}{\frac{\mathbb{d}\lbrack{NO}\rbrack_{th}}{\mathbb{d}t} \approx {\frac{k}{\exp\left( \frac{E_{a}}{RT} \right)} \cdot {\left\lbrack O_{2} \right\rbrack^{1/2}\left\lbrack N_{2} \right\rbrack}}} & (1)\end{matrix}$where k is a numerical constant, exp denotes the exponential function,E_(a) is a positive activation energy, R denotes the ideal gas constantand T is the local temperature.

In order to reduce the quantity of thermally produced nitric oxide, useof a substantially nitrogen-free oxidizer is known. Thus, anoxygen-enriched oxidizer is used instead of air. However, the reductionof the resulting nitrogen oxides is insufficient to meet the regulationsin force.

To further reduce the quantity of nitrogen oxides produced, it is alsoknown, particularly from U.S. Pat. No. 5,522,721 and EP 0 524 880, tocyclically vary the oxidizer flow rate and/or the fuel flow rate fed tothe flame. The ratio between the local instantaneous concentrations ofoxygen and fuel in the flame is accordingly different from thestoichiometry of the combustion reaction. The local temperature isconsequently lower and, according to the equation (1), this causes afurther reduction of the quantity of thermally produced nitric oxide.However, the flow rate variation parameters, such as the amplitude, thefrequency and the phase of the variations of the flow rates, aredifficult to adjust to obtain a satisfactory heating efficiency and alow release of carbon monoxide (CO). In fact, carbon monoxide is toxicand pollutant, and is generated by incomplete combustion when the localinstantaneous oxygen concentration in the mixture is too low compared tothe local instantaneous concentration of fuel.

Another way to obtain a further reduction of the quantity of nitrogenoxides produced consists in injecting a main part of the oxidizer andthe fuel at two locations of the furnace separated from one another by arelatively long distance. A combustion carried out under theseconditions is called “staged” (see for example EP 0 748 981). A smallpart of the oxidizer is also injected close to the fuel outlet tostabilize the combustion conditions. The main part of the oxidizer andthe fuel are then mixed progressively in the spread volume where thejets overlap. In this way, a gap effect is also obtained, between theratio of the local fuel and oxidizer concentrations on the one hand, andthe stoichiometry of the combustion reaction on the other. Furthermore,this stoichiometric gap effect is superimposed on a dilution effect. Thelocal temperature, and consequently the quantity of nitric oxide, arethereby also reduced. However, in this staged combustion configuration,the position of the flame in the vertical direction is particularlyunstable. The efficiency of heating of the charged material isaccordingly reduced and the roof refractories may be damaged.

It is therefore an object of the present invention to propose acombustion method which does not have the abovementioned drawbacks, orin which these drawbacks are reduced.

SUMMARY

Thus, the invention proposes a combustion method for an industrialfurnace, in which two burner assemblies are placed substantiallyhorizontally, parallel to one another and symmetrically about a medianplane passing between the two assemblies. Each burner assemblycomprises:

-   -   a fuel injector;    -   first, second and third oxidizer injectors placed respectively        at increasing distances from the fuel injector.

An oxidizer feed system cyclically distributes a predefined flow rate ofoxidizer among at least some of the second and third injectors of thetwo burner assemblies.

Since the burner assemblies are substantially horizontal, the flameproduced in the furnace is itself contained in a horizontal plane. Inthis way, the heat produced by the flame is efficiently transferred tothe furnace charge, without excessively heating the roof structurearranged above the furnace at a particular location thereof. Prematurewear of the roof structure is thereby avoided.

The oxidizer is therefore introduced into the furnace at three pointsfor each of the two burner assemblies. The first oxidizer introductionpoint is the first injector, which is the closest to the correspondingfuel injector. It serves to cause a first incomplete combustion of thefuel, which is then completed by the oxidizer introduced by the secondand third injectors. The first injector also generally serves tostabilize the combustion conditions at its outlet. For each burnerassembly, the third oxidizer introduction point is the farthest from thefuel injector, and the second oxidizer injector is located at anintermediate distance from the fuel injector between the distances fromthe first and third injectors.

The oxidizer preferably has an oxygen content above 30% by volume, andeven above 70% by volume.

The total oxidizer flow rate introduced into the furnace is distributedamong the first, second and third injectors of the two burnerassemblies. A predefined part of this total flow rate is injected by thesecond and third oxidizer injectors, with a distribution among at leastsome of them which is cyclically variable. The predefined part of thetotal oxidizer flow rate injected by the two second and by the two thirdoxidizer injectors is substantially constant. It may optionally vary,but much more slowly than those of the individual flow rates of thesecond and/or third oxidizer injectors, which are variable. Thus, apredefined fraction of the oxidizer is injected into the furnace by someof the second and/or third injectors at a given time, and is theninjected by the other second and/or third injectors at a later time. Theoxidizer injection obtained by a device according to the invention istherefore alternated among some of said second and/or third injectors.

The cyclic distribution of the oxidizer flow rate among some of thesecond and third injectors of the two burner assemblies is preferablycarried out at a frequency below 1 hertz. The flame oscillation periodin the furnace is then longer than 1 second. The inventors have observedthat such conditions procure particularly stable combustion.

In the staged combustion obtained, the fuel and oxidizer which areintroduced into the furnace are diluted by the recirculation of theexhaust gases in the combustion zone. For this purpose, a main part ofthe oxidizer is introduced into the furnace at a long distance from thefuel introduction locations. By thereby delaying the mixing between thefuel and the oxidizer, the oxidizer is considerably diluted with theambient gases present in the furnace before entering the main combustionzone. However, to stabilize the flame, it is also necessary to introduceoxidizer into the furnace close to each fuel introduction location. Thepart of oxidizer introduced close to the fuel is called primary flow,and that which is introduced at a distance from the fuel is calledsecondary flow.

Advantageously, the oxidizer feed system supplies the first injectorsrespectively of each burner assembly with respective primary oxidizerflow rates substantially equal at any time.

Each burner assembly generates a flame in the furnace, but when the twoburner assemblies are not too distant from one another, their respectiveflames are combined and form a single combustion volume. Such a singleflame is obtained, in particular, when the distance between therespective fuel injectors of the two burner assemblies is shorter than30 times the diameter of each fuel injector. In the rest of thisdiscussion, the term “flame” roughly designates the total volume inwhich combustion takes place, with the understanding that this volumemay be divided into two parts for a large separating distance betweenthe two burner assemblies.

The cyclic variations in the oxidizer flow rate distribution among atleast some of the second and third injectors cause a horizontal shift ofthe flame in the furnace. Depending on the separating distance betweenthe two burner assemblies and the shape of the curves of variations ofthe oxidizer flow rates of the second and third injectors, the shift inthe flame consists of a fluctuation thereof between two positions or anoscillation of the flame between two configurations. In general, thecyclic variations in the gas distribution in the furnace improve thestability of the flame, particularly in the vertical direction, byshifting the flame alternately in a substantially horizontal direction.

Finally, the shift in the flame serves to further improve the heatingpower distribution throughout the volume of the furnace: a heat transferto the furnace charge is obtained, which is more uniform thanks to thetime averaging effect of the heat inputs taking place at each point ofthe furnace.

The invention also proposes a furnace suitable for implementing a methodas described above.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will appear fromthe description of two nonlimiting exemplary embodiments, with referenceto the drawings appended hereto, in which:

FIG. 1 shows the configuration of a furnace suitable for implementingthe invention;

FIG. 2 a is a diagram of the variation in oxidizer flow rates of thefirst, second and third injectors of a furnace as in FIG. 1, accordingto a first embodiment of the invention;

FIG. 2 b shows two configurations of the flame obtained at differenttimes for the flow rate variations shown in FIG. 2 a;

FIGS. 3 a and 3 b correspond respectively to FIGS. 2 a and 2 b for asecond embodiment of the invention; and

FIG. 4 shows various flame configurations corresponding to improvementsof the second embodiment of the invention.

DETAILED DESCRIPTION

For the clarity of the figures, the dimensions of the devices shown arenot in proportion to the actual dimensions. In particular, thedimensions measured in these figures which are associated with distinctactual dimensions are not transposed in the same scale ratio.

FIG. 1 shows a vertical wall 101 of a furnace 100, for example of a rawmaterial melting furnace. The furnace 100 may have batch operation, withdistinct charging, heating and furnace discharging steps, or continuousoperation, with permanent flows of raw material charging and meltdischarging. F denotes the free surface of material charged on the wall101 of the furnace.

The fuel and oxidizer injectors are placed on the wall 101, withsubstantially horizontal respective fluid outlet directions. They arealigned with a horizontal line located at a height h above the line F. his preferably between 250 mm (millimeters) and 550 mm.

The wall 101 is divided into two portions by a vertical median plane P,respectively left, denoted G, and right, denoted D. Injectors arelocated symmetrically on the two wall portions as follows:

-   -   two fuel injectors, referenced 10 _(G) and 10 _(D), are placed        respectively on the wall portions G and D at the same distance        d₁₀ from the median plane P, measured horizontally;    -   three oxidizer injectors, referenced 1 _(G), 2 _(G) and 3 _(G),        are aligned in the wall portion G, respectively at distances d₁,        d₂ and d₃ from the median plane P. The distances from the        injectors of the wall portion G to the median plane P satisfy,        for example, the following equation: d₁<d₁₀<d₂<d₃. The injectors        10 _(G), 1 _(G), 2 _(G) and 3 _(G) are generally located on the        same horizontal line; and    -   three oxidizer injectors 1 _(D), 2 _(D) and 3 _(D) respectively        identical to the injectors 1 _(G), 2 _(G) and 3 _(G) and placed        symmetrically thereto on the wall portion D.

The injectors 10 _(G), 1 _(G), 2 _(G) and 3 _(G) form a first burnerassembly, associated with the left hand portion of the wall 101. Forsimplification, this burner assembly is denoted by G below. Similarly,the injectors 10 _(D), 1 _(D), 2 _(D) and 3 _(D) form a second burnerassembly, denoted by D and associated with the right hand portion of thewall 101.

The fuel introduced into the furnace 100 by the injectors 10 _(G) and 10_(D) may be gaseous or liquid. In the case of a liquid fuel, theinjectors 10 _(G) and 10 _(D) each incorporate a spray nozzle in orderto produce jets of fuel droplets.

Preferably, the distance d₁₀ between the fuel injector of each burnerassembly, 10 _(G) or 10 _(D), and the median plane P is shorter than 15times the diameter of each injector 10 _(G) or 10 _(D), denoted Φ₁₀.Under these conditions, a single flame common to the two burnerassemblies G and D is generated in the furnace 100.

The oxidizer introduced by the injector 1 _(G), 2 _(G), 3 _(G), 1 _(D),2 _(D) and 3 _(D) is a gas normally having an oxygen content above 70%by volume.

Preferably, the third oxidizer injector of each burner assembly islocated at a distance from the fuel injector of said assembly at least10 times longer than the outlet diameter of the third injector. In otherwords: d₃−d₁₀>10.Φ₃, where Φ₃ denotes the outlet diameter of theinjectors 3 _(G) and 3 _(D). Thus, the oxidizer jet of the injector 3_(G), respectively 3 _(D), is sufficiently distant from the fuel jet ofthe injector 10 _(G), respectively 10 _(D), to obtain a stagedcombustion.

All the injectors of each burner assembly are directed substantiallyhorizontally, so that the flame produced is parallel to the surface ofthe melt contained in the furnace 100.

Advantageously, the oxidizer feed system supplies each of the firstinjectors respectively of each burner assembly, that is, the injectors 1_(G) and 1 _(D), with a constant respective primary oxidizer flow rate.The oxidizer feed system is then simplified, in terms of the supply ofinjectors 1 _(G) and 1 _(D). Preferably, the respective flow rates ofthe two injectors 1 _(G) and 1 _(D) are substantially equal:x_(G)=x_(D), denoting by x_(G) and x_(D) the respective flow rates ofthe injectors 1 _(G) and 1 _(D). By way of example, x_(G) and x_(D) eachcorrespond to 10% of the total oxidizing flow rate injected into eachburner assembly.

According to a first embodiment of the invention, described withreference to FIGS. 2 a and 2 b, the oxidizer flow rates of two injectorsplaced symmetrically about the median plane P are equal at any time. Bydenoting by y_(G), y_(D), z_(G) and z_(D), the respective instantaneousflow rates of the injectors 2 _(G), 2 _(D), 3 _(G) and 3 _(D), thefollowing equations are satisfied: y_(G)=y_(D) and z_(G)=z_(D). In otherwords, the oxidizer feed system supplies the second injectorsrespectively of each burner assembly with respective secondary oxidizerflow rates substantially equal at any time, and supplies the thirdinjectors respectively of each burner assembly with respective tertiaryoxidizer flow rates substantially equal at any time. For example, thesupply system of the injectors 2 _(G), 2 _(D), 3 _(G) and 3 _(D) maycomprise two identical distribution boxes assigned respectively to eachburner assembly G and D. These distribution boxes are coupled with acommon variable control member, and each box comprises a mobile wall forseparating the oxidizer flows sent respectively to the second or thirdinjector.

The flame obtained is accordingly centered on the median plane P and issymmetrical about it at any time.

FIG. 2 a shows an example of the variation in flow rates y_(G) and y_(D)on the one hand, and the flow rates z_(G) and z_(D) on the other. Thex-axis shows the time, indicated in seconds, and the y-axis shows thefraction of oxidizer flow rate of each burner assembly which isintroduced by each injector thereof. It is assumed that the totaloxidizer flow rate of each burner assembly G or D is constant, and thatx_(G) and x_(D) are also constant and each equal to 10% of the flow rateof the corresponding burner assembly.

By way of example, y_(G) and y_(D) substantially vary sinusoidallybetween 10% and 50%, and z_(G) and z_(D) vary between 40% and 80%. Theperiod of these variations is 2 seconds. The extreme configurations ofthe flame correspond to the following states:

-   -   state 1, in which y_(G)=y_(D)=10% and z_(G)=z_(D)=80%    -   state 2, in which y_(G)=y_(D)=50% and z_(G)=z_(D)=40%.

The volume of mixture is larger in state 1 than in state 2. According toFIG. 2 b which shows the perimeter 200 of the flame in a horizontalplane passing through the injectors, state 1 corresponds to an extendedflame, both in terms of width and length, and state 2 corresponds to anarrower and shorter flame. For the sake of clarity, the flow rateintroduced into the furnace by each oxidizer injector is shown in FIG. 2b. In state 1, the fuel and oxidizer are more diluted in the flame. Thetemperature is then lower, but a better coverage of the entire surfaceof the charged material is obtained. The heat transfer from the flame tothe furnace charge is then particularly uniform. Conversely, the flameis more concentrated and intense in state 2.

A second embodiment is now described in conjunction with FIGS. 3 a and 3b. This second embodiment corresponds to an alternate oxidizer supplybetween the two burner assemblies. More particularly, the oxidizer feedsystem cyclically distributes a predefined total tertiary oxidizer flowrate said third injectors of the two burner assemblies.

The oxidizer feed system may further supply each of the second injectorsrespectively of each burner assembly with a constant respectivesecondary oxidizer flow rate. A particularly simple implementation ofthe alternate oxidizer feed is thereby obtained. Furthermore, thesecondary oxidizer flow rates may be substantially equal.

The furnace and the burner assemblies used above from the firstembodiment of the invention may be repeated without change for operationwith alternate oxidizer feed. With the same notations and references, wenow have: x_(G)=x_(D)=x/2 and y_(G)=y_(D)=y/2, where x denotes the totaloxidizer flow rate introduced into the furnace 100 by the injectors 1_(G) and 1 _(D), and y denotes the total oxidizer flow rate introducedby the injectors 2 _(G) and 2 _(D). x and y are respectively called thetotal primary and secondary oxidizer flow rates. Similarly, z denotesthe total tertiary oxidizer flow rate, that is, the oxidizer flow rateintroduced by the injectors 3 _(G) and 3 _(D). By way of example, x=10%,y=15% and z=75%, expressed as percentages of the total oxidizer flowrate introduced into the furnace. In general, x and y are substantiallyconstant or vary much slower than the individual injector flow rateswhich vary cyclically.

The oxidizer feed system may be a distribution box connected to theinjectors 3 _(G) and 3 _(D), which has a mobile separating wall placedbetween the oxidizer flows sent respectively to the injectors 3 _(G) and3 _(D). FIG. 3 a shows such an operation, whereby the equationz_(G)+z_(D)=z is satisfied at any time. The y-axis in FIG. 3 b shows thepercentage of the total oxidizer flow rate introduced into the furnace,that is x+y+z. z_(G) and z_(D) each vary between 10% and 65%. The periodof the flow rate variations is also 2 seconds.

The extreme flame configurations now correspond to the following states:

-   -   state 1, in which z_(G)=65% and z_(D)=10%,    -   state 2, in which z_(D)=65% and z_(G)=10%.

The volume of mixture and the flame have symmetrical configurationsbetween the preceding states 1 and 2 (FIG. 3 b). In each of thesestates, the flame is shifted toward the side of the injector 3 _(G) or 3_(D) having the higher oxidizer flow rate. Thus, the flame is shiftedtoward the left side in state 1, and toward the right side in state 2.This sideways fluctuation of the flame stabilizes the height thereof, sothat the flame remains at a substantially constant difference from thefree surface of the charged material on the one hand, and at asubstantially constant distance from the furnace roof on the other.These two distances can then be well controlled, in order to obtain auniform melting process and slow down the degradation of the roofrefractories.

Furthermore, this sideways fluctuation of the flame procures a fairlyuniform heat transfer between the flame and the furnace charge, in ahorizontal direction parallel to the wall 101.

Due to the speed of the oxidizer at the outlet of the injectors 3 _(G)and 3 _(D), the flame is longer on the side of the injector 3 _(G) or 3_(D) having the higher instantaneous oxidizer flow rate. This produces agood average coverage of the furnace surface by the flame. By way ofexample, the oxidizer is expelled by the injectors 3 _(G) and 3 _(D) ata speed of between 20 m·s⁻¹ (meters per second) and 160 m·s⁻¹, forexample 90m·s⁻¹. In general, the average distance of fuel and oxidizer,and the average distance at which combustion occurs, from the furnacewall 101, are commensurately longer as the speed of expulsion of theoxidizer by the injectors 3 _(G) and 3 _(D) is higher.

Furthermore, with each alternation, the high oxidizer flow rateintroduced by one of the two injectors 3 _(G) and 3 _(D) causes asubstantial dilution of the fuel on the side of the median plane P whichcorresponds to this injector. Conversely, the fuel is more concentratedin a zone of the flame offset to the median plane P on the side of theinjector 3 _(G) or 3 _(D) which has the lower instantaneous oxidizerflow rate. This zone is denoted A in FIG. 3 b, for flame perimeters 200corresponding to each of the two states 1 and 2. The zone A hence shiftsat each alternation between two symmetrical positions on either side ofthe median plane P.

Since the flame is richer in fuel in zone A, a larger quantity of sootis produced at this location. Simultaneously, zone A corresponds to thepart of the flame that contributes most to the heat transfer to thecharge at any time.

The existence of such a zone A inside the flame may be favorable orharmful to the material which is being melted, particularly depending onthe chemical behavior of this material when the temperature is notuniform. According to an improvement to the second embodiment of theinvention, the presence of such a zone A can be attenuated orexacerbated by varying the fuel flow rate of the injectors 10 _(G) and10 _(D) at each alternation. For this purpose, a fuel feed systemcyclically distributes a predefined total fuel flow rate among the fuelinjectors of the two burner assemblies.

Advantageously, the fuel feed system is coupled with the oxidizer feedsystem so that the total fuel flow rate is cyclically distributed amongthe fuel injectors of the two burner assemblies in phase with or inphase opposition to the cyclic distribution of the total tertiaryoxidizer flow rate among the third injectors of the two burnerassemblies.

For example, another distribution box may be placed at the inlet of theinjectors 10 _(G) and 10 _(D). This other distribution box has a mobileseparating wall placed between the fuel flows sent respectively to theinjectors 10 _(G) and 10 _(D).

The two distribution boxes, connected to the injectors 3 _(G) and 3 _(D)for the first, and to the injectors 10 _(G) and 10 _(D) for the second,can then be controlled synchronously in phase opposition: the fuel flowrate sent to one of the two injectors 10 _(G) or 10 _(D) is maximal orminimal at the same time that the oxidizer flow rate sent to theinjector 3 _(D) or 3 _(G) on the opposite side is also maximal orminimal. A reinforcement of the zone A is thereby obtained, causing anincrease in the luminosity of the flame close to the outlet of the fuelinjector 10 _(G) or 10 _(D) when the fuel flow rate therein is amaximum. The fuel concentration is leaner on the side of the injector 3_(G) or 3 _(D) for which the oxidizer flow rate is a maximum. Thisincreased depletion causes a shortening of the flame at its furthestpoint from the injectors.

Conversely, the two distribution boxes can be controlled synchronouslyin phase. The fuel flow rate sent to one of the two injectors 3 ₁₀ or 3₁₀ is then maximal or minimal at the same time as the oxidizer flow ratesent to the injector 3 _(G) or 3 _(D) on the same side is also maximalor minimal. The zone A is then diminished and may merge with the overallextent of the flame. Said flame then oscillates between the two left andright hand sides with a higher transverse displacement amplitude.Simultaneously, the flame is elongated, so that the two effects arecombined to obtain an optimal sweep of the entire furnace surface by theflame. This results in a particularly high average heat transfer surfaceto the charge.

The flame perimeters obtained when the fuel flow rate distributionvaries at the same time as the oxidizer flow rate distribution are shownin FIG. 4. The plots 200 a and 200 b correspond respectively tovariations in phase opposition and in phase. The plot 200 corresponds toa constant fuel flow rate distribution, balanced between the twoinjectors 10 _(G) and 10 _(D). It is shown by a dotted line forcomparison. The plots 200, 200 a and 200 b all correspond to identicaltotal fuel and oxidizer flow rates. For the sake of clarity in FIG. 4,only the contour of the flame in state 1 defined above is shown for eachcase.

It is understood that numerous modifications and adjustments to theinvention can be introduced with regard to the embodiments described indetail. Such modifications or adjustments may in particular take accountof particular features, especially geometric, of the furnace in whichthe invention is implemented. Furthermore, the frequency of variation ofthe oxidizer flow rates can be adjusted in a manner known to a personskilled in the art, particularly to obtain a maximum combustion rate andto decrease the quantity of carbon monoxide produced.

It will be understood that many additional changes in the details,materials, steps and arrangement of parts, which have been hereindescribed in order to explain the nature of the invention, may be madeby those skilled in the art within the principle and scope of theinvention as expressed in the appended claims. Thus, the presentinvention is not intended to be limited to the specific embodiments inthe examples given above.

1. A combustion method in which two burner assemblies are placedsubstantially horizontally, parallel to one another and symmetricallyabout a median plane passing between the two assemblies, each burnerassembly comprising: a) a fuel injector (10 _(G), 10 _(D)); and b) first(1 _(G), 1 _(D)), second (2 _(G), 2 _(D)) and third (3 _(G), 3 _(D))oxidizer injectors placed respectively at increasing distances from thefuel injector (10 _(G), 10 _(D)), wherein: a total oxidizer flow rateintroduced into the furnace is distributed among the first, second andthird injectors; a predefined part of the total oxidizer flow rate isinjected by the second and third oxidizer injectors (2 _(G), 2 _(D), 3_(G), 3 _(D)); an oxidizer feed system cyclically distributes thepredefined part of the total oxidizer flow rate among at least some ofthe second and third injectors (2 _(G), 2 _(D), 3 _(G), 3 _(D)) of thetwo burner assemblies, the cyclical distribution being performed by:distributing the predefined part between a portion I and a portion II,portion I being allotted to the second injectors (2 _(G), 2 _(D)) andportion II being allotted to the third injectors (3 _(G), 3 _(D)),wherein the relative distribution of the predefined part betweenportions I and II follows a cyclical pattern, the amount of portion Iallotted to a first one (2 _(G)) of the second injectors (2 _(G), 2_(D)) at any one moment is equal to the amount of portion I allotted tothe other (2 _(D)) of the second injectors (2 _(G), 2 _(D)), and theamount of portion II allotted to a first one (3 _(G)) of third injectors(3 _(G), 3 _(D)) at any one moment is equal to the amount of portion IIallotted to the other (3 _(D)) of the third injectors (3 _(G), 3 _(D));OR the predefined part is divided into first and second portions, thefirst portion is allotted equally between a first one (2 _(G)) of thesecond injectors (2 _(G), 2 _(D)) and the other one (2 _(D)) of thesecond injectors (2 _(G), 2 _(D)), the second portion is distributedbetween a first one (3 _(G)) of the third injectors (3 _(G), 3 _(D)) andthe other one (3 _(D)) of the third injectors, wherein the relativedistribution of the second portion between the first one (3 _(G)) andthe other one (3 _(D)) of the third injectors (3 _(G), 3 _(D)) follows acyclical pattern such that when a fraction of the second portionallotted to the first one (3 _(G)) of the third injectors (3 _(G), 3_(D)) goes up a fraction of the second portion allotted to the other one(3 _(D)) of the third iniectors (3 _(G), 3 _(D)) goes down; and the flowrate of the predefined part is either constant or variable with theproviso that, if the flow rate of the predefined part is variable, theflow rate of the predefined part varies more slowly than the cyclicalchange in flow rates through the second (2 _(G), 2 _(D)) and third (3_(G), 3 _(D)) injectors that is realized by the relative distribution ofthe predefined part between portions I and II and the flow rate of thepredefined part varies more slowly than the cyclical change in flowrates through the first (3 _(G)) and other one (3 _(D)) of the thirdinjectors that is realized by the relative distribution of the secondportion therebetween.
 2. The method of claim 1, in which the cyclicdistribution of the oxidizer flow rate among some of the second andthird injectors of the two burner assemblies (2 _(G), 2 _(D), 3 _(G), 3_(D)) is carried out at a frequency below 1 hertz.
 3. The method ofclaim 1, in which a distance between the respective fuel injectors (10_(G), 10 _(D)) of the two burner assemblies is shorter than 30 times thediameter of each fuel injector (Φ₁₀).
 4. The method of claim 1, in whichthe oxidizer has an oxygen content above 30% by volume.
 5. The method ofclaim 1, in which the third oxidizer injector (3 _(G), 3 _(D)) of eachburner assembly is located at a distance from the fuel injector (10_(G), 10 _(D)) of said burner assembly at least 10 times longer than theoutlet diameter of said third injector (Φ₃).
 6. The method of claim 1,in which the oxidizer feed system supplies each of the first injectors(1 _(G), 1 _(D)) of each burner assembly with a constant respectiveprimary oxidizer flow rate (x_(G), x_(D)).
 7. The method of claim 1, inwhich the oxidizer feed system cyclically distributes a predefined totaltertiary oxidizer flow rate among the third injectors (3 _(G), 3 _(D))of the two burner assemblies.
 8. The method of claim 7, in which theoxidizer feed system supplies each of the second injectors (2 _(G), 2_(D)) respectively of each burner assembly with a constant respectivesecondary oxidizer flow rate (y_(G), y_(D)).
 9. The method of claim 7,in which a fuel feed system cyclically distributes a predefined totalfuel flow rate among the fuel injectors (10 _(G), 10 _(D)) of the twoburner assemblies.
 10. The method of claim 7, in which a flow rate ofthe combined oxidizer cyclically distributed among the third injectorsis constant.